With oil no longer flowing from the Gulf blowout, and green shoots beginning to appear in some patches of marshland where oil from the damaged well made landfall, scientists are beginning to note a few, tiny, early signs of the environment's recovery from the disaster.

They caution that recovery will take from years to decades. Much depends on where the remaining oil and methane from the blowout lurk as they disperse and await the natural processes that break them down.

And if history is any guide, some habitats that look "restored" to the untrained eye may not support the same mix of organisms that thrived in them prior to the blowout.

Still, the federal government has opened roughly one-third of the federal waters in the Gulf it had closed to fishing. At week's end federal officials sounded a conditional all-clear for southern Florida, the Keys, and the state's east coast – a region long concerned that the Gulf's loop current would sweep surface oil into its flow and daub the state's coastal areas with crude. The condition? That the flow from the well remains stanched.

These small bits of encouraging news don't surprise scientists who have long studied the effects of oil spills, says Edward Overton, an environmental chemist and professor emeritus at Louisiana State University (LSU).

Oil spills and blowouts "are horrible. There is carnage everywhere during the event," he says. Because so much of the oil, methane, and dispersants has remained deep in the ocean during the Gulf blowout, "there is damage that you and I can't see, and it won't be obvious for several years, maybe a decade or two."

Still, he adds, once the oil is cleaned, the environment begins to recover fairly quickly.

It's often the case that when researchers and other specialists face an accident that has little or no precedent, the initial tendency is to steel for the worst. Overestimating the potential effects risks the finger-wagging that comes with 20-20 hindsight. Underestimating the effects, however, risks far more.

When an accident pumps 3 million to 5 million barrels of oil into the Gulf, "you can't understate the impact and gravity of the situation," said retired Coast Guard Adm. Thad Allen, who heads the federal response effort, last week during a press briefing.

"I think anybody that classifies this as anything less than catastrophic isn't being realistic. The American people would expect an overabundance of caution," he said. "While we'd all like the area to come back as quickly as possible, the fact is that in the history of our country we've never put this much oil in the water."

Several factors argue for the potential of a relatively quick recovery for some sea species that may have been affected, some researchers say.

The federal and state bans on fishing reduced the human pressure on fish stocks this year. And at least on or near the surface, an expected increase in bacteria munching on the remaining oil could increase the food available for organisms higher up the food chain, making the area an attractive feeding ground for sea birds and fish. Oil does not work its way up the food chain as other compounds, such as mercury, do.

Yet much remains unclear about conditions under the surface, including the number of animals for whom the oil and dispersants already have proved fatal.

"Right now, the body count on the spill is pretty minimal," Dr. Shirley acknowledges, referring to the number of fish, birds, and marine mammals known to have died from contact with oil. But he says he expects the number to rise significantly as researchers conduct follow-up studies.

"We know from Prince William Sound that only a very small percentage of the oiled animals ever made it to the surface," he says, referring to research conducted in the aftermath of the Exxon Valdez spill in Alaska in 1989.

"With Exxon Valdez we didn't find out the full effects of the spill – and we still don't know all of them – until years after the spill had been cleaned up and the Rush Limbaughs of the world were telling us it was pristine again," he says.

Marine organisms that are likely to take the biggest hits are large and reach reproductive maturity at a relatively late age, Shirley says. These include turtles and marine mammals such as porpoises and whales. And most of these will never reach shore, but sink as they succumb either to the direct toxic effects of oil or through indirect effects, such as compromised immune systems, he says.

That said, "I never was a doom-and-gloom guy; I know that the Gulf will recover," he adds. "Many of the commercially important species will rebound rather quickly," particularly shrimp and oysters in least-affected oyster grounds.

Underwater unknowns

Concerns continue to surround the enormous amount of oil remaining thousands of feet below the surface. There, it's much harder to measure, track, and even attribute effects to the oil and methane scientists have detected.

The deep-sea ecosystem is used to dealing with natural oil seeps known to dot the bottom. But the impact of millions of barrels of atomized oil, methane, and dispersant at those depths are poorly known.

One of many aspects scientists are eager to track is the effect of the oil on the amount of oxygen in the water.

Scientists have detected low oxygen levels within the subsea clouds of oil and methane they've detected so far. But the low levels are still significantly higher than the levels needed to declare the water "hypoxic."

Bacteria consume oxygen to break down the oil and methane. Between the cold temperatures at those depths, and the bacteria's own needs, the already-slow breakdown process could decelerate further, some researchers suggest.

To this point, no one has been able to put numbers to the amount of oil microbes may have eaten, notes Samantha Joye, a marine scientist at the University of Georgia who has been leading a team of undersea plume hunters. Nor, she says, does anyone know whether bacteria break down the dispersants.

Indeed, throughout the blowout, researchers have pointed to the almost overwhelming list of unknowns in an unprecedented off-shore oil accident.

Thus, even for those who use past experience to foresee the Gulf's recovery, it's a cautious optimism.

"There's going to be a big damage-assessment study of this spill over the next 10 years," says LSU's Overton. "But if my assessment is close to right, it means our environment will come back fairly quickly and people's lifestyles will not be wiped out, that they'll be able to make a living off the northern Gulf and enjoy the recreational benefits of this body of water. We'll have to see."